there are 5 hosts (logstash-forwarder > redis > logstash-center > elasticsearch < Kibana), when I push an message to logstash-forwarder, redis always receives 2 messages, I try to delete the keys of list of redis, but it seems cannot work fine, how to solve the problem, following are my details setting

Check what files you have in /etc/logstash/conf.d. I suspect you have an extra configuration file with a redis output so that each message is sent to Redis twice.

Note: logstash-forwarder is the name of a program separate from Logstash. The configuration snippet you've shown is for Logstash. To avoid confusion, don't say logstash-forwarder when you mean Logstash.